The line was built by the Great Western Railway and engineered by Isambard Kingdom Brunel as a dual track line using a wider 7 ft (2,134 mm) broad gauge and was opened in stages between 1838 and 1840. The alignment was so level and straight it was nicknamed ‘Brunel’s Billiard Table’. It was supplemented with a third rail for dual gauge operation allowing standard gauge 4 ft 81⁄2 in (1,435 mm) trains to also operate on the route in stages between 1854 and 1875. Dual gauge was introduced as follows: London to Reading (1 October 1861), Reading to Didcot (22 December 1856), Didcot to Swindon (February 1872), Swindon to Thingley Junction, Chippenham (June 1874), Thingley Junction to Bathampton (16 March 1875), Bathampton to Bristol (June 1874), Bristol station area (29 May 1854). The broad gauge remained in use until 1892. Evidence of the original broad gauge can still be seen at many places where bridges are a wider than usual, or where tracks are ten feet apart instead of the usual six.

The original dual tracks were widened to four track in various places between 1877 and 1899. Paddington to Southall (1 October 1877), Southall to West Drayton (25 November 1878), West Drayton to Slough (1 June 1879), Slough to east side of Maidenhead Bridge (8 September 1884), Maidenhead Bridge to Reading (4 June 1893), Reading station (1899), Reading to Pangbourne (30 July 1893), Pangbourne to Cholsey and Moulsford (?), Cholsey and Moulsford to Didcot (27 December 1892), Various short sections between Didcot and Swindon, and at Bristol.

Following the Slough rail accident in 1900 when five passengers were killed improved vacuum braking systems were used on locomotives and passenger rolling stock and Automatic Train Control (ATC) was introduced in 1908.

At the outbreak of World War I in 1914, the Great Western Railway was taken into government control, as were most major railways in Britain and were reorganised after the war into the "big four" companies, of which the Great Western Railway was one. The railways returned to direct government control during World War II before being nationalised to form British Railways in 1948.

The line speed was upgraded in the 1970s to support the introduction of the InterCity 125 (HST).[2]

In 1977 the Parliamentary Select Committee on Nationalised Industries recommmended considering electrification of more of Britain's rail network, and by 1979 BR presented a range of options that included electrifying the GW Main Line from Paddington to Swansea by 2000.[3] Under the 1979–90 Conservative governments that succeeded the 1976–79 Labour government the proposal was not implemented.

In August 2008 it was announced that a number of speed limits on the relief lines between Reading and London have been raised so that 86% of the line can be used at 90 miles per hour (140 km/h),[4] however the time allowed between stations for trains running on the relief lines has been reduced in the December 2008 timetable to improve timekeeping.[5]

Between London and Didcot there are four tracks, grouped by speed with the "relief" lines on the north side of the "main" lines. The only stations that have fast line platforms in use are at London Paddington, Slough, Maidenhead, Twyford, Reading and Didcot Parkway. Between Didcot and Royal Wootton Bassett there are a series of passing loops lines to allow fast trains to overtake slower ones. This section is also signalled for bi-directional running on each line but this facility is usually only used during engineering working or when there is significant disruption to traffic in one direction.

The line speed is 125 miles per hour (201 km/h). The relief lines from Paddington to Didcot are currently limited to 90 miles per hour (140 km/h) as far as Reading, and then 100 miles per hour (160 km/h) to Didcot. Lower restrictions apply at various locations. It is one of only two Network Rail-owned lines to be equipped with the Automatic Train Protection (ATP) system, the other being the Chiltern Main Line.

Swansea railway station is to undergo renovation which will include an enlarged concourse, a new entrance, a new partition wall between concourse and platforms together with a new cafe and more shops.[10][11]

A major renewal programme is underway from bases at Reading and Taunton.

As part of Crossrail the Great Western was already planned to be electrified from Airport Junction to Maidenhead but, following a number of announcements and delays the government announced in March 2011 that the line would be electrified between London and Cardiff together with the section linking Bristol Parkway and Bristol Temple Meads.[12][13] In July 2012, the UK Government announced that the final portion of the Great Western from Cardiff to Swansea would be electrified.[14]

Network Rail plans to install European Rail Traffic Management System (ERTMS) in-cab signalling on the Great Western line:[15][16] this is a pre-requisite for the new Super Express trains to run at 140 mph (225 km/h).[17] Some or all of the resignalling work will be done during the electrification work.[15]

Further capacity improvements are also scheduled at Swindon, adding to recent changes and the new Platform 4.

Crossrail services are planned to terminate at Reading. Some of the current suburban services into London Paddington are planned to be transferred to the new Crossrail service, which will in turn free up some surface-level capacity at London Paddington.[15]

By 2016, there are plans for a direct rail link from Swindon to London Heathrow Airport.[18] There are also calls for the reintroduction of a station at Corsham[19] due to recent growth of the town. The original station was closed to passengers in 1965.

Other more distant aspirations include resignalling and capacity improvements at Reading; the provision of four continuous tracks between Didcot and Swindon (including a grade-separated junction at Milton, where the down (westbound) relief line switches from the north side of the line to the south); and resignalling between Bath and Bristol to enable trains to run closer together. A local group is campaigning for the reopening of Saltford station between Bath and Bristol, to coincide with electrification.[20]

Access to Heathrow Airport from the west remains an aspiration and there is a proposed future link to Heathrow Airport directly from Reading under the Heathrow Airtrack scheme which would use a route south of the Great Western main line. Plans for electrification of the line will make it easier to access Heathrow from Reading given that lack of electrification between Reading station and Heathrow Airport Junction near West Drayton station was a limiting factor.[15]

Network Rail intends to replace the ATP system with ETCS – Level 2[21] from 2017 to 2035 along with the introduction of the new IEP trains.

Signalling Solutions is to resignal the 12 miles from Paddington to West Drayton, including the Airport branch, as part of the Crossrail project.[22]

The Slough rail accident in June 1900 occurred after an express train from Paddington to Falmouth Docks ran through two sets of signals at danger, and collided with a local train heading for Windsor. The collision resulted in the death of 5 passengers, and 35 were seriously injured.

The Ealing rail crash occurred on 19 December 1973 when a train from Paddington to Oxford derailed: ten passengers were killed and 94 injured. The cause of the accident was a loose battery box cover on the BR Class 52 "Western" locomotive, that struck line side equipment, causing a set of points to move under the train.

The Southall rail crash occurred in 1997 when an InterCity 125 operated by Great Western Trains from Swansea to London Paddington failed to stop at a red signal and collided with a freight train entering Southall goods yard. Seven people were killed and 139 were injured. The incident severely damaged public confidence in the safety of the rail system. It was found that the train's AWS was faulty, and the driver had been distracted (he had bent down to pack his bag). Great Western Trains was fined £1.5 million for violations of health and safety law in connection with the accident.